Bug 1021981 - [email] Update toast display to 2.0 visual redesign

This styles the toast according to the styling in comment 9 of the bug. The button dimensions listed in the PDF in the zip file did not seem correct, so I just eyeballed the size to match the design display. Notes on the pull request:

* message_list: one of the other 2.1 features is to remove the refresh button from the sync failed case, so I did that here. It means we do not need to ship a retry icon for the toast, and is the reason the only image asset type in this pull request is for undo.
* toaster.html was changed to use a div wrapping the button, just to give a larger tap target when the user tries to tap on the button. With just the button as the target, the user could have tapped just outside the button but it would have just dismissed the toast without executing the button action.
* toaster.js: an `actionClass` option to the toast was added to allow styling the action button based on the action type. Since we removed the retry option in this pull request, only 'undo' is used now, but it allows us some flexibility in the future if we introduce a new button actions.
* The styling uses flexbox for alignment. When the button is not in use, the p tag for the message takes the whole space.
* The csslint xlint change was because the font-face: 0 was added to the button, and the csslint complained about too many font-face declarations. This was done so that the text can be used for accessibility. The fix for that longer term is probably to break up common.css into smaller pieces, or perhaps move to using a `a11y-text` class name to do that work. That would be a bigger change though, and probably should be done as a stand-alone ticket.

I used these commands via the app-manager console to trigger the display of toast in different forms:

Shows plain text message with no retry/undo:

    require('toaster').toast({ text: navigator.mozL10n.get('compose-attachment-still-working') });

Shows retry button:

    require('toaster').toast({ text: navigator.mozL10n.get('toaster-retryable-syncfailed') });

Shows undo button:

    require('toaster').toast({ text: navigator.mozL10n.get('toaster-message-read', { n: 1 }), actionLabel: navigator.mozL10n.get('toaster-undo'), actionClass: 'undo',action: function(){ alert('yes'); } })